This invention relates to radar search and guidance means and has particular reference to a system using synthetic aperture radar and an interferometric antenna to provide relative range and azimuth information.
A prior art search has revealed by way of U.S. Pat. No. 3,812,493 that it is old in the art of bistatic passive radar apparatus to employ an interferometric antenna to receive signals from a radiating source and via reflection from a target to calculate range and bearing of the target. Such apparatus however depends on the fortuitous illumination of radar significant targets by emitters and detection of the resulting echos at the radar as a means of ranging on such emitters with the use of conventional interferometry for absolute angle measurement. Such a system does not provide for relative observance of a weapon and a target by an airplane as an aid to getting the former to the latter.
U.S. Pat. No. 3,085,242 was also brought to light by a search of the prior U.S. patented art as were some specific foreign publications showing an electronic tracking system where phase differences between antenna pairs in a mutually bisecting antenna baseline system are used to determine direction to multiple transponder targets whose transponder replies may be used for range determination and are sequenced amongst targets. Again there is no ability of determining by a combination of an interferometer antenna and synthetic aperture radar the relative distance and angle between a weapon and a target both a distance from the determining means.
Another prior art U.S. Pat. No. 3,858,211 was discovered that set forth an apparatus whereby slant range and relative bearing to an intruder is determined by secondary surveillance radar in cooperation with transponder-bearing aircraft. In contrast to the interferometric synthetic aperture radar of this invention this reference teaches apparatus which involved ground-based traffic control keyed to the concept of synchronization with a 360.degree. radar search pattern. There is no means provided to obtain both relative range and azimuth information.
U.S. Pat. No. 3,856,237 teaches use of a search radar in a weapon to illuminate a target for an aircraft with which it is synchronized by a transponder signal so that calculations may be made and the weapon guided by the aircraft. However, in contrast to this invention, this patent teaches only a means to illuminate a target from a weapon with apparatus carried by the weapon. Such systems render weapon detection and location much more probable than the passive weapon system of this disclosure. It should be noted that similar prior art teachings with likewise similar distinct differences with this invention are found in U.S. Pat. Nos. 3,074,062 and 3,698,811.
There has also been noted the teaching of U.S. Pat. No. 3,739,378 which is to provide apparatus to specifically determine absolute range and azimuth to an airborne target. In this invention the stated object is to use apparatus to measure relative range and relative azimuth between a cooperative (transponder) missile (weapon) and a non-cooperative or passive ground target.
As this prior art illustrates it is well known to use a weapon such as a guided missile to provide information of its own location and that of a target to a remote station to, at the remote station, guide the weapon to the target. This is also shown by U.S. Pat. No. 3,415,465. Therefore, this general technique is standard and as such is used in IFF and Air Traffic Control. However this art is not illustrative of how one can use a passive weapon (passive in the sense that it does not probe for a target with means that would lead an enemy to believe a threat is presented by the weapon) transponder to, with target mapping by a remote station, obtain information of the weapon's relative range and azimuth to the target.